1. Field of the Invention
The present invention relates to the preparation of higher chloromethanes by chlorination of methyl chloride, and, more especially, to the simultaneous such preparation of select amounts of the three higher chloromethanes CH.sub.2 Cl.sub.2, CHCl.sub.3 and CCl.sub.4.
2. Description of the Prior Art
Conventional processes for the preparation of higher chloromethanes by chlorination of CH.sub.3 Cl, as a practical matter, result only in the production of CH.sub.2 Cl.sub.2 and CHCl.sub.3. The CCl.sub.4 co-product constitutes less than 15% of the total production and is thus considered to be only a by-product of the reaction.
The production of CCl.sub.4 is typically carried out by so-called "Tetra-per" (carbon tetrachloride/perchloroethylene) processes from chlorinated residues and/or saturated or olefinic hydrocarbons having two or three carbon atoms. However, such techniques lead to the generation of a not inconsiderable amount of heavy by-products. In addition, they consume large amounts of energy.
Because of these disadvantages of conventional processes, the preparation of CC1.sub.4 from chloromethanes originating from the chlorination of CH.sub.3 Cl would be of interest, especially since the most recent improvements in this process permit free radical chlorination in the liquid phase at a low temperature, using a chemical or photochemical initiator. Such processes are economical due to the high yields obtained and the simplification of the separation operations, the amount of troublesome by-products being small.
However, in the current state of the art, the simultaneous production of prescribed amounts, which may be close to one another, of the three higher chloromethanes does not proceed without giving rise to problems in the conventional apparatus which consists of a reactor for the chlorination of CH.sub.3 Cl followed by downstream columns for purifying and separating the various products of reaction. Such a system lacks flexibility. In fact, in this type of unit, the principal constraint is the adjustment for the respective productions of CH.sub.2 Cl.sub.2 and CHCl.sub.3 ; the amount of CCl.sub.4 simultaneously produced is a function of the CH.sub.2 Cl.sub.2 -CHCl.sub.3 proportion effectively obtained and the operating conditions. This production of CCl.sub.4 can only be low with respect to that of the other two chloromethanes.
The production of prescribed amounts, which may be closed to one another, of the three higher chloromethanes in conventional apparatus comprising a single chlorination reactor must necessarily be balanced by the total amount of investment required for profitable operation. In fact, in this type of unit, the adjustment in the flexibility is obtained by limiting the degree of conversion of CH.sub.3 Cl and, if necessary, by appropriate recycling of CH.sub.2 Cl.sub.2. This manifests itself in a significant over-dimensioning in the entirety of the apparatus concerned, and concomitant marked increase in operating costs. Moreover, the productivity of a given reactor, all things otherwise being equal, is sensitive to the composition of the higher chloromethanes produced. It is known that the rate of chlorination of a chlormethane substantially decreases as the number of chlorine atoms contained in the molecule increases: a reactor having a productivity of 100 for respective compositions of CH.sub.2 Cl.sub.2, CHCl.sub.3 and CCl.sub.4 of about 45/45/10% by weight would only have, all things otherwise again being equal, a productivity of about 65 for a distribution of 37.5/47.5/15 and of 30 for 20/45/35. "Productivity" is defined as the amount of chlorine reacted per unit volume of the reactor for a fixed level of unreacted chlorine at the outlet of the reactor.